The invention relates to a vehicle brake system having at least two brake pressure lines, via which a master brake cylinder is connected to wheel brake cylinders, as defined hereinafter. Such brake systems are known in a great number of vehicles. To generate the brake pressure and thus the braking action, a master brake cylinder is acted upon with pressure with the aid of a foot pedal. This pressure is then passed on to wheel brake cylinders via the corresponding brake pressure lines, causing a closure of brake calipers, for instance.
To preclude wheel locking and a resultant impairment of performance and controllability of a vehicle, modern brake systems today additionally have built-in anti-lock or anti-skid control systems, also known as ABS, which regulate the brake pressure acting upon the wheel brake cylinders. To this end, there are sensors on the wheels that recognize locking of a particular wheel and report it to a control unit. In the event of anti-lock control, a magnetic valve incorporated into the brake pressure line is switched to the blocking position, and brake fluid is withdrawn from the wheel brake cylinders, for instance via a return pump, thereby relieving these wheel brake cylinders. In the normal operating state, in which there is no current to these so-called ABS magnet valves, the valves are switched on, so that there is continuous communication between the master brake cylinder and the wheel brake cylinder. Only during the anti-lock control is this communication severed, or else from the wheel brake cylinders some other connection is opened for pressure reduction.
It is also known that a dynamic shift in axle load occurs during braking. This depends on the magnitude of the braking deceleration and on the static distribution of the load on the vehicle and the height of its center of gravity. During straight ahead braking, the front wheels are loaded more heavily than the rear wheels. For safety reasons it is known to use a plurality of brake circuits, the distribution of brake circuits can be selected as separate for the front and rear axles, as an example, so that if the front axle brake circuit fails, for instance, then the rear axle can still be braked, and vice versa. Diagonal brake circuit distributions are also known, in which one front wheel and a diagonally opposed rear wheel are each assigned to the same brake circuit. In most vehicles the brakes are designed such that the best performance during braking results for medium deceleration. In major deviations from mean values, either the rear wheel can lock, causing the vehicle to spin out, or the front wheel are locked, so that the vehicle can no longer be steered.
To prevent overbraking of the rear axle, particularly, and the resultant skidding of the vehicle, in many vehicles having hydraulic brake systems, a pressure reducer is built in the lines between the rear wheel brake cylinders and the master brake cylinder. It is intended to limit the brake pressure acting upon the rear wheel to a certain value.
In vehicles that are equipped with an anti-lock or anti-skid brake system, contrarily, if the pressure reducer is dispensed with, then although locking of a rear wheel is prevented, still the occurrence of brake slip at the rear axle earlier than at the front axle worsens the braking stability. To prevent this, and to assure vehicle stability in the event of failure of the anti-lock control system, and to achieve the brake performance required by law, pressure reducers are also provided in anti-lock brake systems. However, it is very time-consuming and labor-intensive to install the known pressure reducers.